<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        console.log(num); // undefined
        var num = 10; 

        fn1(); // 可以
        function fn1() {
            console.log(11);
        }

        fn2(); // 不行
        var fn2 = function(){
            console.log(22);
        }
    </script>
</body>
</html>